This invention relates to a digital recording and playback system in which digital signals of non-return-to-zero inverted format are recorded at high density using a plurality of stationary magnetic heads.
In digital recording systems known in the art, audio signals are translated to PCM signals and then organized into a sequence of blocks each comprising digital words of data and synchronization and digital words of error detection and correction. The binary digits of the organized signal are of non-return-to-zero (NRZ) format. However, due to the fact that the clock component of the NRZ signal tends to vary with the contents of the data word, the clock information would be lost if a sequence of zero's continues. A transformation technique is currently employed to transform the original non-return-to-zero (NRZ) signal into a form which contains a sufficient amount of clock information to make it highly likely that binary digits are reproduced in correct clock timing.
On the other hand, the use of conventional 3.81-mm width tapes and conventional stationary heads having a minimum gap of 0.3 micrometers imposes limitation on recording density although high tape-speed operations would solve the density problem.
Known transformation techniques include modified frequency modulation (MFM), 3-position modulation (3PM), and group-coded recording (GCR). Since MFM or 3PM transformed, self-clocked signals have a wide frequency spectrum containing a substantial amount of energy the very low frequency range near DC and in the high frequency range, conventional recording systems having a typical tape speed of 7.1 cm/sec suffer from cross-talk due to leakage flux between adjacent recording heads, with the result that the reproduced waveform is contaminated with noise and the error rate increases considerably in the high frequency range. Moreover, these problems are compounded by the fact that due to the differential characteristic the magnetic head produces a ternary signal rather than a binary signal, making it difficult to achieve high density recording.